Yeah, it may be more difficult to see on gunmetals, but definitely noticeable on black/silvers. But it depends on a variety of factors too such as the amount already built up and how much water droplets are still on the wheel

That's the rust from the rotors after water has hit them. Really annoying especially after the hard work I put into cleaning the rims. So do the particles actually fly off right after you hit the brakes a little and then use the leaf blower?